Glutathione reductase participates in preventing lipid peroxidation by oxygen radicals which results in cellular damage. The brain is among the organs most susceptible to cadmium-induced lipid peroxidation. The mechanism of free radical generation by Cd2+ is not well understood, but it is known that Cd2+ is an inhibitor of glutathione reductase. In this study, inhibition kinetics of the brain glutathione reductase by Cd2+ was investigated. Sheep brain enzyme (11,000-fold purified) was used for this purpose. The data were analyzed by a nonlinear curve fitting program. It was found that the inhibition was competitive with respect to oxidized glutathione and uncompetitive with respect to NADPH. Inhibition constants were found as 12.3 and 9.4 mu M, respectively. These findings might contribute to the understanding of the mechanism of lipid peroxidation by Cd2+ in brain. (C) 1995 Academic Press, Inc.